CMOS Image Sensor Pixel Exposure Control for Wide Dynamic Range
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Solution Overview
Problem
CMOS image sensors have a limited dynamic range, failing to capture all details in uncontrolled lighting environments, as they struggle to effectively 'underexpose' in bright areas and 'overexpose' in dark areas without increasing pixel size or control circuit complexity.
Innovation Solution
A high dynamic range CMOS image sensor with a special exposure control circuit that adjusts photodiode exposure time for each pixel, determining if the charge is approaching saturation and resetting it to prevent saturation in bright areas while maximizing exposure in dark areas, using exposure time adjustment circuitry and Correlated Double Sampling techniques.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the full-well capacity of the sensor's photodiodes is increased to improve dynamic range, then the quality of data captures in brighter areas is improved, but the sensitivity of the sensor degrades
Solution Approach 1:
The patent applies dynamics by making the exposure time variable for each pixel based on its local lighting conditions. The exposure control circuit dynamically adjusts integration time - extending it for dark areas to capture more photons, and shortening it for bright areas to prevent saturation. This dynamic adaptation allows the sensor to optimize both sensitivity in dark regions and avoid saturation in bright regions, resolving the contradiction between dynamic range and sensitivity.
Solution Approach 2:
The patent implements local quality by allowing different exposure times for different pixels within the same image frame. Each pixel's exposure time is independently controlled based on its local scene brightness, rather than applying a uniform exposure time to the entire sensor array. This local optimization enables each pixel to operate at its optimal sensitivity level, capturing details in both dark and bright areas simultaneously.
2Reliability
If the exposure time is extended to capture more light in dark areas, then sensitivity is improved, but the photodiode saturates in bright areas
Solution Approach 1:
The patent applies local quality by implementing pixel-specific exposure time control. The exposure control circuit evaluates the local lighting conditions for each pixel and assigns an appropriate exposure time from a set of available integration times. Pixels in dark areas receive extended exposure times to maximize photon collection and sensitivity, while pixels in bright areas receive shorter exposure times to prevent saturation, thereby achieving both goals simultaneously across different regions of the image.
Solution Approach 2:
The patent uses dynamics by introducing multiple discrete exposure time levels that can be selectively applied during a single frame period. The system dynamically transitions between different integration time settings based on real-time assessment of local scene brightness, allowing the sensor to adapt its sensitivity characteristics spatially and temporally to match the varying lighting conditions across the scene.
3Measurement precision
If independent exposure time adjustment is implemented for each pixel, then dynamic range is improved, but device complexity increases
Solution Approach 1:
The patent applies universality by designing a shared exposure control circuit that serves all pixels in the sensor array. Rather than providing dedicated control circuitry for each pixel, a single multi-functional control circuit evaluates local brightness conditions and manages exposure time assignment for the entire array. This universal control approach enables independent exposure adjustment for each pixel while avoiding the complexity overhead of per-pixel control circuits.
Solution Approach 2:
The patent uses segmentation by dividing the exposure control function into discrete, manageable components: a brightness evaluation stage, an exposure time selection stage, and an implementation stage. The control circuit processes pixels in organized groups or sequences, applying exposure time adjustments in a structured manner. This segmented approach to control simplifies the overall circuit design while maintaining the capability for independent pixel-level exposure management.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach allows for superior image data quality over a wide dynamic range by independently adjusting exposure times, effectively 'underexposing' in bright conditions and 'overexposing' in dark conditions, without increasing pixel size or control circuit complexity, thereby optimizing photodiode charge collection.
Implementation Method 1
pixels which transform photons coming from a photographed scene or other optical image source into a large number of corresponding voltage signals
Data Source
AI summary
A CMOS image sensor uses a special exposure control circuit to independently adjust the photodiode exposure (integration) time for each pixel in a pixel array to obtain non-saturated photodiode charges for each pixel. Exposure time adjustment involves extrapolating a pixel's final photodiode charge using an intermediate photodiode charge measured after a predetermined portion of an exposure frame period. If the intermediate photodiode charge is, e.g., over 50% of the photodiode's full-well capacity after half of the exposure frame period, then saturation is likely and the photodiode is reset to integrate only during the remaining time. If not, then the photodiode integrates over the allotted exposure frame period. Data indicating the length of the exposure portion is stored as analog data on the memory node of each pixel, and readout of the final photodiode charge is performed using Correlated Double Sampling (CDS) techniques.